The invention pertains to laser light sources and particularly to vertical cavity surface emitting lasers. More particularly, the invention pertains to long wavelength surface emitting lasers.
A vertical cavity surface emitting laser (VCSEL) may include a first distributed Bragg reflector (DBR), also referred to as a mirror stack, formed on top of a substrate by semiconductor epitaxial growth techniques, an active region formed on top of the first mirror stack, and a second mirror stack formed on top of the active region. The VCSEL may be driven by a current forced through the active region, typically achieved by providing a first contact on the reverse side of the substrate and a second contact on top of the second mirror stack. The first contact may instead be on top of the first mirror stack in a coplanar arrangement.
VCSEL mirror stacks are generally formed of multiple pairs of layers often referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being lattice matched to the semiconductor substrate. For example, a GaAs based VCSEL typically uses an AlAs/GaAs or AlxGa1-xAs/AlyGa1-yAs material system wherein the different refractive index of each layer of a pair is achieved by altering the aluminum content in the layers. The number of mirror pairs per stack may range from 20 to 50 to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. The larger number of pairs increases the percentage of reflected light.
In many VCSELS, conventional material systems perform adequately. However, new products are being developed requiring VCSELs which emit light having longer wavelengths. VCSELs emitting light having a longer wavelength are of great interest in the optical telecommunications industry because of the low fiber dispersion at 1310 nanometers (nm) and the low fiber loss at 1550 nm. As an example, a long wavelength VCSEL may be obtained by using a VCSEL having an InAlGaAs/InAlAs active region. When an InAlGaAs/InAlAs active region is used, an InP/InGaAsP material system lattice-matched to the InP substrate may be used for the mirror stacks in order to achieve a lattice match. The lattice matching between the substrate and the layers need to be substantially close to ensure a true crystal film growth.
In the InP material based system, it is practically impossible to achieve a suitable monolithic DBR-based mirror structure because of the insignificant difference in the refractive indices available in this lattice matched material system. As a result, many layers, or mirror pairs, are needed in order to achieve useful reflectivity. Useful reflectivity must generally be 99.8 percent or greater. Numerous attempts have been made to address this problem including a wafer bonding technique in which a DBR mirror is grown on a separate substrate and bonded to the active region. This technique has had only limited success and also the interface defect density in the wafer fusion procedure causes potential reliability problems. Other approaches to making satisfactory long wavelength VCSELs have been fraught with one problem or another. For instance, lattice matched InP based mirrors used for 1550 nm VCSELs have a host of problems in growth, processing, and optical performance. The low index contrast of lattice matched InGaAsP and InAlGaAs leads to the requirement of extremely thick (ten microns or thicker) DBRs of 45 or more mirror periods. The AlGaAsSb or AlGaPSb systems may be difficult to grow by MOCVD, and with good contrast, may still require at least 25 mirror pairs to achieve adequate reflectivity for VCSEL operation.